This invention relates to the recovery or reclaiming of substantially pure metal from metal matrix composites and, more particularly, to a process for recovering substantially pure aluminum from aluminum matrix composites and also recovering the reinforcing particulates.
Aluminum matrix composites are widely used where high strengths and other enhanced properties are required in castings. As materials for reinforcing the aluminum matrix, a variety of non-metallic particulates are typically used, e.g. ceramic particles, such as silicon carbide, alumina, etc. in amounts in the range of 10 to 30% by volume.
In processes in which the aluminum matrix composites are utilized, there is a proportion of waste or scrap materials. The scrap from these metal matrix composites represents a serious disposal problem since metal matrix composites cannot be incorporated into standard alloys on account of the danger of contamination by the reinforcing particles present. For instance, a metal matrix composite containing even a very low concentration of ceramic particulates could have disastrous effects if mixed with metal destined for thin gauge rolled products, such as can stock.
The injection of a reactive gas or an inert-reactive gas mixture into molten metal is a commonly used technique for the removal of contaminants, including solids. Such systems are described for instance in Bruno et al, U.S. Pat. No. 3,839,019, Szekely, U.S. Pat. No. 3,743,263, Withers et al, U.S. Pat. No. 4,634,105 etc. These prior systems are based upon injecting gas in the form of small discrete bubbles into a melt and these small bubbles are intended to float or dewet a substantial proportion of any solid contaminants contained in the molten metal and remove other contaminants, such as dissolved hydrogen, in the melt.
While such processes are quite effective, they are not capable of removing all reinforcing particulates from the matrix such that the metal portion can be reused as a pure material.
The use of a molten salt bath in a rotary furnace is a more usual way of removing non-metallic inclusions from aluminum scrap. However, a large quantity of salt (e.g. about 20-50% by weight) would be needed to remove all the ceramic particulate from a metal matrix composite. This would not be environmentally acceptable.
It is the object of the present invention to provide a simple process for removing all solid particles contained in metal matrix composite scrap materials.